1. Field of the Invention
The present invention relates to resonators. More particularly, the present invention relates to couplings among a plurality of resonators. Still more particularly, the present invention relates to coupling between or among non-adjacent resonators.
2. Description of the Prior Art
Non-adjacent coupling between resonators in RF filters is a widely established technique to achieve transmission zeros at desired frequencies and thus establish sharp rejections in certain frequency ranges without increasing the number of resonators. Most of the real world applications require non-symmetrical frequency response; i.e., one side of the frequency band has much higher rejection requirements than the other and thus the ability to place transmission zeros arbitrarily at desired frequencies can produce both symmetric and non-symmetric frequencies. This very ability allows us to reduce filter sizes while minimizing insertion loss and at the same time increasing rejections in desired frequencies. Some of the techniques to couple non-adjacent cavities are to bring non-adjacent cavities physically closer, but this approach may not always be possible or be impractically difficult due to geometry constraints.